The computerized image processing is a technology widely used in the industry. Before being processed by a computer, image data shall be input into a computer system with a digital image input device. Digital image input devices include: digital video camera, digital still camera, optical scanner etc. Among them, the optical scanner provides the function to input in a digitized format image data which are recorded in two-dimensional recording media. The optical scanner is inexpensive and has became a standard office equipment.
In the conventional optical scanner, the charge coupler device is used as the sensing element. The charge coupler device picks up the image components carried by the light beams entering into it, converts them into electronic signals and outputs the signals. Due to the fact that the charge coupler device is sensible to the brightness (gray level) of the input light, special arrangements shall be necessary when the charge coupler device is used as a input device for colored images.
U.S. Pat. No. 4,809,062 disclosed a color optical scanner with a roller. In this invention, a roller with filters of the three original colors, each filter for one original color, is at a up-stream position relative to the charge coupler device and is driven by a stepping motor. The filters with each original color pass by the charge coupler device sequentially. At any time point at most one filter passes by the indicence position of the charge coupler device through which the incidence light beam is filtered and only the original color components of the input light is allowed to enter the charge coupler device and is sensed by the charge coupler device. In the following time period the roller rotates to a second angle so that the next original colored filter enters into the scanning position. The original color components of the incidence light beam will be sensed by the charge coupler device. And so on.
Under such design, charge coupler devices which are sensible to the brightness of the incidence light, may be used to scan colored image data.
U.S. Pat. No. 5,122,911 relates to a filter device for a optical color scanner. The filter device has a longitudinal frame in which four filtering windows are provided. Among the four windows three are provided for three color filters, each for one original color. A driving control device controls the relative position between the charge coupler device and any one of the three original color filters. Within any time period only one of the color filter locates at the incidence position to the charge coupler device. When one scanning (e.g. scanning a page of a document) is completed, another filter is driven to proceed to the scanning position. Another scanning is then processed. This approach is called the sequential scanning and provides the function of color scanning for image data.
In another conventional art, the "color charge coupler device" or "color CCD" is employed in the color scanning. Here, the so-called "color CCD" means CCD's coated with colored filter or colored membrane. Due to the fact that CCD's are fine components, coating of the color filter or color membrane involved complicated processed. Effective method to steam color filter onto CCD's is still not industrially feasible. To be simplified, only one color is coated on the surface of a CCD chip.
In the application of the "color CCD's", three CCD's, each coated with an original color membrane, are employed where each CCD serves to scan image data components of one original color. Digitized color image data are obtained after the combination of the three original-colored image data components, processed by computer software or electronic circuits.
Among the above-described three approaches, the color CCD approach is the most popular one. This is mainly because the small dimension of the CCD helps to minimize the dimension of the system. It is however pointed out that several shortages are incurred from using the original-color membrane as the filtering means.
FIG. 1 illustrates the spectrum of light as filtered by filters of the three original color, sensed by CCD's. As shown in the figure, within the wave length between 400 nm to 700 nm, the spectrum of light as filtered by blue colored filter distributes between 400 nm to 500 nm, by green filter distributes between 480 nm to 580 nm and by red filter distributes between 560 nm to 700 nm. Overlaps take place in between the original colors. In other words, light as filtered by a filter of an original color may be not of an original color; Cross talk with other original color(s) is found. This cross talk becomes noise in the processing of image data. Further processing to eliminate the noise is thus required in order to obtain correct image data of an original color.
Due to the above-described fact, image data as scanned would require corrections, resulted at time-consuming processing and high costs for the image processing system.